1. Technical Field
The present invention relates to a method and apparatus which provide compensation for excursion of a rotary shaft relative to its housing.
More particularly, the invention relates to an improved combined centrifugal and magnetic fluid seal structure in which the centrifugal seal combines with a fluid film bearing by increasing the fluid pressure in the bearing as a function of the centrifugal seal fluid pressure to maintain a proper spacing between a cartridge seal member and a shaft collar.
Still more particularly, the present invention relates to an improvement of the plural fluid magnetic-centrifugal seal described and illustrated in U.S. Pat. No. 4,335,885 (Heshmat), the subject matter of which is expressly incorporated herein in its entirety by this reference. In providing such improvement, the present invention makes use, in part, of the technology of hydrostatic bearings and hybrid (combined hydrostatic and hydrodynamic) bearings described and illustrated in U.S. Pat. No. 3,708,215 (Wilcock et al), the subject matter of which is also expressly incorporated herein by reference in its entirety.
2. Discussion of the Prior Art
There are many applications, such as pumps, compressors, and the like, where hermetic sealing of the rotatable shaft in a non-contacting, non-wearing manner is not only desirable but essential during both zero or low speed operation and during high speed operation. Because of the sealing requirement in high speed applications, and in order to reduce wear, it is desirable that the seal be non-contacting in nature. A prior art seal which meets these requirements is described and illustrated in the aforementioned Heshmat patent. Specifically, the invention described therein relates to a combined centrifugal and magnetic seal structure employing separate, different viscosity fluids for use during separate magnetic seal and centrifugal seal operating modes of the structure, whereby each seal compliments the other at different rotational speeds. The plural fluid seal makes it possible for the design parameters of each stage, although coacting over a complete speed range, to be substantially independent of one another, so that optimized design criteria may be employed in the construction of the two cooperating seals. However, for certain applications, such as with contra-rotating shafts and/or for a rotor with excessive axial motion, the sealing structure disclosed in the Heshmat patent may not fully accomodate the more extreme axial shaft excursions. Consequently, particularly during high speed shaft rotation, the rotating and non-rotating parts may sometimes be brought into contact resulting in increased wear and reduced life of the parts of the seal structure. It is known in the prior art to employ fluid film bearings between mutually rotating parts in order to minimize contact between the parts. There are three primary types of fluid film bearings, namely, the hydrodynamic bearing, the hydrostatic bearing and the hybrid bearing. The hydrodynamic bearing has essentially no limit on its fatigue life. If the bearing materials are chosen to be compatable with the lubricant employed, so that there is an absence of corrosion and chemical attack, and if the bearing design is such that high-speed rubbing contact is avoided during operation, operating life of this type of bearing may be considered to be infinite. However, particularly for applications where the axial thrust load is variable, such bearings experience a period of metal-to-metal rubbing during extreme load variations.
The hydrostatic bearing, like the hydrodynamic bearing, has essentially no limit on its operating life, provided it is designed so that no rubbing occurs during certain periods of operation. However, the hydrostatic bearing, like the hydrodynamic bearing, suffers under conditions where the thrust load is variable over a relatively wide range and may result in metal-to-metal contact and undesirable wear.
The hybrid bearing, which is basically a hydrostatic bearing, has a hydrodynamic capability built into it. This hydrodynamic capability adds additional load capacity.
It is desirable to utilize the fluid film bearing technology in connection with the combined fluid seal structure of the aforementioned Heshmat patent in order to provide an efficient seal combined with compensation for shaft excursion at higher rotational speeds. However, the existing fluid film bearing technology, as described above, is not capable of preventing the undesirable contact and wear between the sealed parts under all operating conditions.